1. Field of the Invention
The invention relates to a local area network operating in the asynchronous transfer mode (ATM) comprising a plurality of stations which are coupled each via a network interface, which interface comprises a coupling device and a control arrangement and transmits cells via the coupling device between send and receive ring lines, to at least one station and at least one control arrangement.
2. Discussion of the Related Art
A local area network operating in the asynchronous transfer mode (ATM) is known from the publication "Ohne Chips nichts los--Standard-Chips f ur ATM-Systeme sind im Kommen" by Bernd Reder, Elektronik 1/1993, pages 66 to 75. That paper discusses the current situation of the development of integrated circuits for applications in local area networks, which networks operate in the asynchronous transfer mode (ATM). Each station is then connected to the ring lines of the local area network via the network interface. Switching devices (switches) are used in the network interfaces to control the ATM cell stream.
When an asynchronous transfer mode is used in a system, payload, for example, telephone, video or sound signals, is sent in blocks of a fixed length through arrangements for digital signal processing. By a block of a fixed length is meant a cell which contains a given number of bytes (for example, 53 bytes). Each cell contains a header field having a length of, for example, 5 bytes, and an information field which accommodates the payload and has a length of, for example, 48 bytes. In such a header field are available routing indicators, error recognition data and control data. Routing indicators are meant to be understood as trunk identifiers and link identifiers. The link identifier, also referenced VCI (Virtual Channel Identifier), contains the description of the destination of the cell in the system. To transmit a cell, a virtual channel is made available in response to the virtual channel identifier. As a rule, a VCI is changed each time a switching point is reached. A trunk group of various virtual channels is referenced a virtual path. A virtual path is identified by the trunk identifier. Such a virtual path is called a virtual path identifier (VPI). Cells are assigned to consecutively determined periods of time. The length of such a period of time depends on the clock rate used as a basis of the transfer components. If no payload is available, empty cells i.e. cells without payload, are transmitted in such a period of time. Cells containing payload are referenced payload cells.
A network interface of the local area network defined above may be coupled, for example, via a plurality of send and receive ring lines, to further network interfaces and also to a plurality of stations. A station may be, for example, a videophone, a personal computer, a workstation or a telephone. For control purposes there is at least one control arrangement available which usually comprises a bus system, a bus controller, memory modules, interfaces, an arithmetic-logic unit and so on. The control arrangement can transmit, for example, speech information accommodated in cells via an interface to a further unit, or be used for setting up or taking down connections. Such a control arrangement, however, cannot operate at the maximum available bit rate of the local area network. A cell stream that has a bit rate of, for example, 155 Mbit/s cannot be processed in the control arrangement with currently available technology. Nowadays a cell stream having a bit rate of about 10 Mbit/s can be processed in the control arrangement.